1. Field of the Invention
This invention relates generally to the field of geophysical prospecting and more particularly to an apparatus and method for generating acoustic pressure pulses in water by implosion which are well adapted for marine seismic prospecting.
2. Description of the Prior Art
A considerable amount of attention has already been focused in the prior art of marine seismic energy systems on the generation of so-called acoustic waves in water by implosion. Existing devices and techniques of this character create a gaseous bubble in the water by known means which then implodes or collapses due to hydrostatic forces to provide such an acoustic pulse. Generally, the bubble is created either by injecting air or other gas into the water or by rapidly separating a pair of contiguous submerged rigid body surfaces to establish a partial void in the intervening space. The latter approach has the advantage that the gaseous phase occupying the bubble prior to implosion is condensable water vapor and hence offers the least resistance to bubble collapse, thereby enhancing the peak pressure pulse amplitude obtainable and reducing the oscillatory pulses caused by expansion and contraction of the non-condensable gas. The present invention relies upon the formation of such a condensable water vapor bubble but the method and apparatus are fundamentally different.
Cavitation, or creation of a low pressure vapor phase within a body of water, is a phenomenon which accompanies the movement of rigid objects in a fluid environment. In general, its presence is considered undesirable because it produces a drop in efficiency and because the energy of a collapsing cavitation bubble tends to erode or pit the surface of the body against which it forms. In a known form of marine seismic gas exploder exemplified by U.S. Pat. No. 3,401,770 to Kilmer et al, the presence of cavitation is recognized. Here a gas explosion confined within a combustion chamber in the exploder drives a bottom mass in a downward direction to deliver a primary compressive pulse to the water. When the mass reaches the limit of its allowable travel and reverses direction due to reaction forces, cavitation occurs at its under surface. The collapse or implosion of such a cavity generates impulsive pressures perceived as a secondary pulse. Prior devices of this character have been concerned to attenuate or minimize the effect of such a secondary pulse so as to avoid interference with the interpretation of the primary pulse.
Attention is also directed for comparative purposes to U.S. Pat. No. 3,818,440 to Dransfield et al which is directed to an apparatus supporting a marine seismic gas generator of the type under consideration beneath the surface of the water. In this apparatus, the entire weight of the generator is transmitted to the float through a hollow vertical firing tube which is fixed to a recoil piston slidable within an outer cylindrical housing. A gas explosion confined within the combustion chamber formed between the lower surface of the piston and the bottom mass of the cylinder drives the latter downwardly to generate a compressive pulse of interest. Chains are attached to the housing from the float to prevent a pendulum effect in towing. However, they are not under tension so as to support the weight of the housing and they do not initially restrain, damp, or limit its downward movement. With this type of suspension for the seismic generator, experiment has shown that although a cavitation pulse occurs, its peak amplitude is not more than twice that of the primary pulse.
With these aspects of the prior art in mind, the inventors have devised a method and apparatus for supporting a marine seismic gas exploder of the type described wherein the initial compressive pulse is highly attenuated and the secondary cavitation pulse is emphasized to an extent sufficient to greatly increase the difference in their relative peak amplitudes. As a consequence, the cavitation pulse appears substantially alone in the resultant acoustic wave form.